(a) Field of the Invention
The present invention relates to an objective for an endoscope and, more particularly, to an objective for an endoscope employing relay lenses (endoscope arranged to transmit the image of object, which is formed by the objective, by using relay lenses).
(b) Description of the Prior Art
There is a known optical system for an endoscope employing relay lenses which is arranged that the image of object focused by an objective O is formed in the inside of field lens F as shown in FIG. 1 and aberrations thereof are corrected favourably. This type of endoscope optical system employing relay lenses has features as described in the following: (1) the exit pupil of the objective O is transmitted to the relay lens system comprising the relay lenses R.sub.1, R.sub.2 and R.sub.3 by using the field lens F, (2) the objective is arranged as a retrofocus type objective so that positive curvature of field is caused in the image formed by the objective so as to thereby cancel negative curvature of field caused by respective relay lenses constituting the relay lens system in order to thereby eliminate curvature of field of the endoscope optical system as a whole, and (3) a cemented doublet having a cemented surface R, which has negative refractive power, is provided as one of positive lens components constituting the objective O, which is arranged as a retrofocus type objective, so that coma of the endoscope optical system as a whole is corrected favourably.
However, the known endoscope optical system shown in the above has disadvantages described in the following. Firstly, dispersion in the field angle occurs in the course of manufacture of said endoscope optical system. Said dispersion in the field angle is caused by dispersion in the action to transmit the pupil by the field lens F. That is, dispersion in the action to transmit the pupil by the field lens F is caused by dispersion in the airspace between the objective O and field lens F, dispersion in the airspace between the field lens F and relay lens system, dispersion in the thickness of field lens, etc. and, consequently, dispersion is caused in the field angle of the endoscope optical system.
The second disadvantage of the known endoscope optical system employing relay lenses is that curvature of field is not yet corrected satisfactorily favourably. Curvature of field caused by the relay lens system is proportional to the number of relay lenses (R.sub.1, R.sub.2, R.sub.3, etc.) which constitute the relay lens system. Therefore, even when negative curvature of field caused by the relay lens system is favourably corrected by means of positive curvature of field caused by the objective in case that the number of relay lenses is three, for example, said negative curvature of field cannot be corrected satisfactorily and remains undercorrected in case that the number of relay lenses is five.